Indeed that solve the symptoms. I discover this was also documented here:

https://wiki.debian.org/PowerPC/FAQ#How_do_I_get_graphics_working.3F

[...]
image=/boot/vmlinux
        label=Linux
        read-only
        initrd=/boot/initrd.img
        append="radeon.agpmode=-1 modprobe.blacklist=ams"
[...]

I am not sure what the 'ams' thingy is doing, but I am using it over here.

The funny thing is that dragging window and moving them around makes the
pointer and the window out-of-sync. Still usable though after a while.

If AGP is simply not supported on PowerPC, it would be nice to have a clearer
message (or at least a warning).

Thanks